1. Field of the Invention
The present invention relates to the separation of articles having different specific gravities and more particularly to a method for separating defective semiconductor devices.
2. Description of the Prior Art
The identification and separation of defective semiconductor devices has long been a problem in the semiconductor manufacturing field and many solutions have been proposed.
A substantial portion of the cost of a finished semiconductor device is attributable to the cost of packaging and therefore substantial losses are experienced when defective devices are packaged. It is most important that defective devices be identified and removed prior to packaging so as to reduce the overall cost of manufacturing.
Several methods have been developed for identifying and removing defective devices. Test equipment has been developed for identifying defective devices wherein an entire wafer having many devices formed thereon may be tested. The defective devices may be marked for subsequent removal after the wafer is scribed and broken into separate dice.
Heretofore, the defective devices on a wafer have been hand marked with an identifying material such as ink so that the defective devices may be manually removed after the wafer is broken into individual dice. In an advanced system, the defective devices were marked with a magnetic ink so that they may be subsequently segregated by passing the dice beneath a magnet which would attract and pick up the defective dice having the magnetic ink marking.
In a more sophisticated system that eliminates the need for hand marking of the defective devices, the semiconductor wafer is placed in a deplating solution and subjected to an electric potential that reverse biases the PN junctions in the devices. Thus, defective devices that are shorted or have excessive leakage will have their metal contacts deplated. Deplating of the metal contacts from the defective devices effectively reduces the specific gravity of the die on which the device is formed since the heavier metal is removed and only the lighter silicon remains.
After the deplated wafer is scribed and broken into individual dice, the dice are poured into a tubular container filled with a bromoform solution having a specific gravity adjusted to a level between that of the silicon dice and the dice having metal contacts. Theoretically, the deplated dice would float in the bromoform solution while the heavier dice having metal contacts would sink to the bottom of the tubular container. The method did not meet with practical success since circulating currents tend to develop in the bromoform solution so that some heavy dice would be carried to the surface by the circulating currents. Thus, the prior art method resulted in excessive numbers of acceptable devices being discarded and defective devices escaping detection.